Fish exhibit an enormous variety of reproductive patterns. There is external and internal fertilization, simultaneous and sequential hermaphroditism as well as gonochorism, and an extremely widespread occurrence of parasitic reproductive behaviour among males. In most fish species there is a great size range of reproductive males, setting the stage for divergent, intraspecific reproductive patterns and an unparalleled concentration of alternative male reproductive phenotypes. Recent theoretical, empirical and comparative evidence suggests that adaptations to sperm competition in fish might be responsible for some of the most intriguing examples of reproductive design known.